The presence of dungeons. The occaisonal dungeon crawl should be fundamental to any d&d game. I don't use dungeon crawls in every adventure my self when I DM, but once every few levels I have a good old fashion dungeon crawl.
A lot of other things in the poll are mechanical things, things that happen outside of the game world. D&D is an ideal that goes beyond the rules.
At its core its about slugging your way through room after room in a dungeon, till you meet the dragon in the middle, slaying it and taking all its loot.
PS I'm glad its not a multi choice poll, I like having to make a definitive decision about what I value most.